In LTE (Long Term Evolution) and successor systems of LTE (referred to as, for example, “LTE-advanced,” “FRA (Future Radio Access),” “4G,” etc.), D2D (Device to Device) technology is under study, whereby user terminals communicate with each other directly, without involving radio base stations (see, for example, non-patent literature 1).
In inter-terminal direct communication (D2D communication), user terminals carry out D2D discovery to find other user terminals that are communicable. In D2D discovery, the network allocates periodic uplink resource groups as D2D discovery resources, semi-statically. The user terminals allocate discovery signals to D2D discovery resources and transmit them. Also, the user terminals find other communicable user terminals by receiving discovery signals transmitted from other user terminals.